Diel variability in transcription of the structural gene for glutamine synthetase (glnA) in natural populations of the marine diazotrophic cyanobacterium Trichodesmium thiebautii

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Abstract
A 360-bp fragment of the Trichodesmium thiebautii glutamine synthetase gene ( glnA ) was amplified from DNA isolated from oceanic populations of this ecologically important, diazotrophic cyanobacterium. The cloned fragment showed high homology to the type 1 glutamine synthetase (GS) enzymes of other cyanobacteria and was used to probe the in situ, temporal variability in transcription of Trichodesmium GS. Three distinct phases in the diel pattern of glnA mRNA abundance were observed. These appear to be correlated with the temporal variability in carbon and nitrogen fixation rates and the effect that these physiological processes have on the size of cellular pools of the primary end-products of N assimilation. Our data show that Trichodesmium thiebautii GS is regulated at the level of transcription and support the hypothesis that GS expression in this organism is subject to similar controls as those established for heterocyst-forming cyanobacteria.

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